Semiconductor wafers such as silicon and gallium arsenide wafers and various types of packages (which may be collectively referred to as a “cut object,” hereinafter) are manufactured in a large diameter state and these are cut and separated (diced) into small element pieces (referred to as “chips,” hereinafter), which are individually released (picked up) and thereafter transferred to a mounting step as the subsequent step. During this operation, a cut object such as a semiconductor wafer undergoes respective steps of dicing, washing, drying, expanding, picking-up and mounting in a state in which the cut object is attached to a dicing sheet that comprises a base film and a pressure sensitive adhesive layer.
Here, in the base film of the above dicing sheet, a problem of blocking may occur. That is, when the base film is wound up, overlapped parts of the base film are in close contact with each other, and when the base film is wound off, feeding thereof cannot be performed and/or troubles may occur in the feeding. Moreover, when the base film is used as a part of a dicing sheet, the base film may come into close contact with a dicing table to cause transport errors.
In addition, in the above dicing step, cracks (chipping) may occur at cut surfaces of chips due to insufficient fixing of a cut object and the like. Such chipping may deteriorate the bending strength of chips themselves and trap air into an IC package in which the chip or chips are enclosed, and package crack may readily occur. Thin semiconductor wafers are recently made and as the thickness is reduced, the possibility of occurrence of chipping as the above becomes high.
According to the commonly-used full-cut dicing as a specific method for the dicing step, a rotating round blade is used for cutting a cut object. In the full-cut dicing, to ensure that the cut object to which the dicing sheet is attached can be completely cut over the entire surface, the pressure sensitive adhesive layer may also be cut beyond the thickness of the cut object and a part of the base film may further be cut.
During this operation, dicing debris comprising materials that constitute the pressure sensitive adhesive layer and base film may arise from the dicing sheet, and the obtained chips may be contaminated with the dicing debris. One form of such dicing debris is filament-like dicing debris that attaches onto a dicing line or to an area near the cross-section of each chip separated by dicing.
If the chip is enclosed in a package while the filament-like dicing debris as the above remains on the chip, then the filament-like dicing debris attached to the chip will be decomposed by heat used for the enclosing and the thermally decomposed substance may destroy the package and/or cause operational failure in a device obtained. Such filament-like dicing debris is difficult to remove by washing and the yield in the dicing step will thus be considerably reduced due to generation of the filament-like dicing debris. Therefore, when a dicing sheet is used to perform dicing, it is needed to prevent the generation of filament-like dicing debris.
To suppress the generation of such dicing debris, Patent Literature 1 discloses an invention which uses, as the base film of a dicing sheet, a polyolefin-based film irradiated with 1 to 80 Mrad of electron rays or γ (gamma) rays. In this invention, it appears that a resin that constitutes the base film is crosslinked through the irradiation with electron rays or γ rays thereby to suppress the generation of dicing debris.